Linking Defect-Controlled Grain Growth and Band-Edge Optical Response in Chymosin-Assisted Pechini-Derived CeO2−δ Nanoparticles

We investigate how grain growth, strain relaxation, and vacancy chemistry shape the near-edge optical response of nanocrystalline CeO2-δ prepared by a chymosin-assisted Pechini route from nitrate-citrate precursors. Rietveld line-profile analysis shows that phase-pure CeO2-δ forms after calcination between 400 and 1000 °C. Over this range, the average crystallite size increases from ≈3.4 to ≈57 nm, while the microstrain decreases from 0.79% to 0.05%, with size-strain scaling consistent with interface-controlled grain growth that follows a normal growth law with exponent m=2 and activation energy Q≈155 kJ mol-1. Raman spectroscopy tracks the sharpening of the F2g mode and the fading of defect-related bands, X-ray photoelectron spectroscopy reveals a nonmonotonic evolution of the surface Ce3+ fraction and separates lattice from adsorbed oxygen species, and electron paramagnetic resonance detects vacancy-bound Ce3+ polarons that weaken at high temperature. Diffuse-reflectance UV-Vis spectra show a modest blue shift of the apparent band gap from Eg≈2.78 to 2.95 eV as crystallites coarsen, while the Urbach energy Eu follows the Ce3+ content and sub-gap tailing. The structural, spectroscopic, and optical results together map out a quantitative connection between grain growth, vacancy populations, and near-edge optical properties in CeO2-δ nanoparticles.